1. Field of the Invention
The present invention relates to image processing devices for displaying digital images, image processing methods, and computer programs for the same. In particular, the present invention relates to image processing devices for enlarging digital images to cope with an increase in resolution and an increase in the size of displays, and image processing methods, and computer programs for the same.
More specifically, the present invention relates to an image processing device for performing on-screen display, which includes icons and characters, on a large display, an image processing method, and a computer program for the same. In particular, the present invention relates to an image processing device for enlarging an on-screen display area while maintaining the display quality, an image processing method, and a computer program for the same.
2. Description of the Related Art
With the current increases in the resolution of displayed images and in the size of displays, processing for enlarging digital images has become a significantly important technique.
The digital images can be mainly categorized into the following two types.
One type is natural images captured with image pickup devices, such as cameras, and recorded or transmitted. The other type is artificial images, such as graphics and character texts, generated with computers.
Methods for enlarging or reducing images using an interpolation filter based on a sampling theorem, such as bilinear interpolation or cubic interpolation, are generally employed for natural images. These image processing methods can provide high-quality enlarged images, regarding natural images that have multilevel data and that include, in principle, a noise.
On the other hand, when artificial images are enlarged using the above-described interpolation filter, edges of characters or the like are dulled, due to which high-quality enlarged images may not be obtained. In particular, for bilevel images, such as character texts, enlargement processing is often performed using the nearest neighbor interpolation in order to avoid the blur resulting from such dulled edges. However, the jaggy (serration observed on a contour of an image) becomes problematic in terms of visual sensation in the enlargement processing using the nearest neighbor interpolation.
In addition, recently more and more complex video images have been handled in artificial images. In such a case, a mismatch undesirably occurs in a combined image of an enlarged character text and another artificial image if the character text is enlarged appropriately only using data matching or the like.
On-screen display (OSD) of an operation screen for screen setting that appears on a display can be given as an example of artificial images to be displayed over a digital image. The OSD function is used as a user interface to media that stores various kinds of image information, and is executed in a format that indicators, such icons or characters representing a device status and an operation condition, are superimposed on a main image. In such a manner, the OSD function allows an operation mode of the device to be displayed as an icon or a device operation guidance to be displayed.
For example, digital cameras are widespread as media that store image information. Digital cameras provide a mode for reproducing and displaying image data stored in a memory card and a mode for displaying through images to be captured. It is known that digital cameras combine image data with character data, representing a counter value, a date, and a photographing condition, on an LCD (Liquid Crystal Display) panel, which is a display device, to perform on-screen display (see, for example, Japanese Unexamined Patent Application Publication No. 11-146234).
In addition, an on-screen controller has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 7-162750). From the viewpoint that on-screen images should be displayed in a relatively easy-to-see size and at a relatively easy-to-see position on a display, the on-screen controller includes means for enlarging or reducing a character or a pattern to a given size, means for displaying the character or the pattern at a given position, and an operation unit having a key for enlarging and reducing the character or the pattern to a given size and a key for moving the character or the pattern to a given position. The on-screen controller allows the character or the pattern to be displayed in the given size and at the given position through operations performed on the keys.
Additionally, the noticeability of on-screen images can be improved by increasing the size of displayed icons related to user settings. For example, a display-equipped electronic apparatus has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 2005-301992). The display-equipped electronic apparatus detects a change in an apparatus status or a change in a setting of an operation condition, assigns an icon indicating the apparatus status or the operation condition for which the change is detected, and performs on-screen display in a shape different from the icon assigned before.
As described above, since opportunities for enlarging artificial images superimposed on natural images are increasing mainly in products, such as digital cameras, obtainment of enlarged artificial images having a quality that is the same as high-resolution multilevel natural images is desired. Hitherto, techniques for performing enlargement processing on image data having relatively less grayscale levels, such as character texts of artificial images, have been suggested.
For example, an outline font generating method has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 5-94171). In this method, character fonts of both of basic characters and enlarged characters are stored beforehand, and several kinds of pattern matching are performed on the enlarged character patterns to generate an enlarged character. This method undesirably requires a mass memory to store the font data. In addition, since this method includes two processing paths of enlargement processing and interpolation processing, which are generating an enlarged character from basic character font data and then performing interpolation on this enlarged character, a processing time and a cost for hardware increase.
In addition, a method for scaling bilevel images using the piecewise polynomial interpolation has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 8-63592). However, this method undesirably requires a ROM storing a vast number of patterns, otherwise the quality of enlarged images becomes low with a small number of patterns. Additionally, the method requires complex arithmetic processing, which results in an increase in a processing time and an increase in a cost of hardware.
Furthermore, a smoothing method performed at the time of enlarging characters and figures has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 9-305755). In this case, complex arithmetic processing is needed, which results in an increase in a processing time and an increase in a cost of hardware.
Moreover, a font generating system that employs a genetic algorithm has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 2004-4302). In this case, complex arithmetic processing is needed, which results in an increase in a processing time and an increase in a cost of hardware.
All of the methods described above basically target at bilevel graphic images, but are incapable of coping with text enhancement, such as outlines and shadowed characters.
Additionally, a method for enlarging characters using pattern matching has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 2006-3420). Although this method allows a target pattern to be enlarged to a high quality image, an unnatural pattern may be caused when a complex artificial image is combined with a background image.